Quote Originally Posted by seneschal.the
Not sure if there is a trigger for just that.

Speculation:

You could possibly work around it by having a check like the mongols/timruids datewise, and if a settlement is not rebel/specific faction then spawn Unholy Hordes of Pancakes thing.
Thanks for the advice, but that probably won't work with my idea
(What I'd like to do is have the Kingdom of Jerusalem emerge after the first succesful crusade against Jerussalem).